This project is based on the preliminary findings that an oxothiolane class of nucleosides (+/-)-BCH-189 and its analogues have shown potent antiviral activity against human hepatitis B virus. We have synthesized an enantiomerically pure (-)-L and (+)-D-BCH-189 from L-gulose and D- mannose, respectively, and discovered that (-)-L-BCH-189 is the most potent isomer against human hepatitis B virus (HBV). Furthermore, the (-)-L-BCH-189 was found to be significantly less toxic than the (+)-D- BCH-189. The superior anti-HBV activity of (-)-L-BCH-189 may be explained based on the fact that (+)-D-BCH-189 is susceptible to deamination by deoxycytidine deaminase while (-)-L-BCH-189 is completely resistant to the deaminase under the similar conditions. Thus, this application proposes the extension of this exciting preliminary findings to synthesize various pyrimidine and purine analogues of oxathiolane L- nucleosides for exhaustive structure-activity relationship studies against HBV. Additionally, assymmetric synthesis of dioxolane L- nucleosides as well as their C-nucleoside analogues will be synthesized. In vitro antiviral activity of the synthesized compounds will be evaluated in the 2.2.15 cells derived from HepG2 cells that were transfected with a plasmid containing HBV. Additionally, biochemical, molecular biological and toxicological (in vitro) studies will be performed with the (-)-L-BCH-189 as well as other promising drug candidates. Furthermore, in collaboration with the National Institute of Allergy and Infectious Diseases, a certain promising drug candidate will be studied in vivo woodchuck hepatitis B model to assess the anti- HBV as well as the toxicity. The long term goal of this application is to discover and develop clinically effective and safe antiviral agents for HBV which is currently lacking despite enormous problems in the world.